Drawn and ironed seamless beverage cans, whether fabricated from aluminum or steel, customarily are reduced in diameter proximal to the open end prior to attaching the lid or top. This reduction in diameter near the top end or open end is done to achieve a reduction in the quantity of material that is required to fabricate the lid. The percentage in material saving and the potential cost savings becomes apparent when it is realized that the thickness of the material for the lid may be 0.013 inches or more, whereas the thickness of the wall material may be only 0.005 to 0.008 inches thick and the savings in material is a function of the square of the reduction in diameter.
Therefore, it is advantageous to reduce the open end of a drawn and ironed beverage can to as small a diameter as can be reasonably achieved and as small as will leave sufficient space for a pull-tab opener. However, the extremely thin walls of beverage cans present difficulties in the necking operation, namely localized buckling of the material inwardly rather than uniform compressive yielding of the material in conformity to meeting progressively reduced diameters in the necking die.
The development of drawn and ironed aerosol cans has also presented a need for large percentage reductions in the open ends of the cans. However, attempts to obtain a large percentage reduction in the open end of seamless, drawn and ironed containers, such as beverage cans and aerosol cans, has resulted in wrinkling or localized buckling.
It has been customary to utilize a punch portion of the die to minimize this localized buckling and to maximize the percentage of reduction in diameter that can be achieved without wrinkling. This general principle is embodied in FIGS. 7-10 of Saunders, U.S. Pat. No. 3,995,572. However, it should be realized that the punch must be retractable from the open end of the container; so it cannot support the open end of the container as the material is deformed inwardly to prevent localized buckling. Instead, it can only prevent localized buckling inwardly of the reduced diameter that is being formed.
This tendency toward buckling during necking operations, combined with the extreme thinness of the material in beverage cans, limits percentage reductions to approximately 4.3 percent in steel containers having a wall thickness of 0.005 to 0.008 inches. Attempts to obtain larger percentages in reduction of diameter by the use of additional forming steps have resulted in a general crumpling of the shoulder or transition portion when the objective has been to both further reduce a previously reduced diameter portion and to reform a previously formed transition portion.
Saunders, in the aforementioned patent, has avoided both the localized wrinkling and shoulder crumpling problems by teaching the forming of a plurality of reduced diameter portions to arrive at a greatly reduced diameter of opening.
Hilgenbrink, in FIG. 4 of U.S. Pat. No. 3,786,957, teaches a die construction for supporting the open end and a first reduced diameter portion of the die while reforming a longitudinal portion of the first reduced diameter portion into a second reduced diameter portion that is both smaller in diameter and shorter than the first reduced diameter portion. Thus both Saunders and Hilgenbrink resort to a plurality of reduced diameter portions to avoid the problems of localized wrinkling and shoulder cumpling occurring during a second or subsequent necking or forming step.
While Hilgenbrink and Saunders have made advances in the art by providing containers that achieve the required reduction in diameter of the opening by a plurality of reduced diameter portion, their solution is not entirely satisfactory for use with beverage cans because the plurality of reduced diameter portions results in an appreciable loss of container volume for a given length and diameter of a container. Therefore, for containers where the number of fluid ounces in a container has been firmly established, as has been done with regard to carbonated soft drinks, and where the size of the containers is limited by automatic vending machines, the advantages of a material saving in a reduced diameter top are largely offset by the loss of standard volume capacities and/or standard container sizes.
In contrast to the prior art, the present invention achieves large reductions in the open end of the container while minimizing the number of reduced diameter portions, providing a savings both in metal and cost of the lids or tops, maintaining standard volumetric capacities in standardized sizes of containers, and minimizing the total number of forming steps.